A look at the upcoming UHA760 USB DAC and Amp
Here's a look at my latest design, the UHA760. My goal with this model is to provide a high-end USB DAC and amplifier combo geared for those who, like me, listen mostly to source material recorded at 44.1-kHz (ripped CDs, Spotify, etc). The UHA760 can handle USB audio rates up to 16-bit/48-kHz. It includes an asynchronous sample rate converter (ASRC) which upsamples the digital audio to 192-kHz. The upsampling process, while not intended to create or replace audio signal content, does have some nifty benefits: because it is an asynchronous process, it isolates the DAC chip from jitter on the USB interface; it also shifts the digital filter in the DAC away from the audio band, allowing for a higher performance external filter. In addition to the ASRC, the UHA760 includes digital volume control and adjustable crossfeed with bypass.
There are numerous DAC designs on the market with upsampling capability, but it's not always clear what upsampling is or why it might be desirable. The upsampling process creates interpolated digital audio samples which sort of "fill in" the space between the original samples. Suppose the original sample rate is 48 kHz. The output of the SRC, running at 192 kHz, would have four samples for every one sample in the input signal.
Although it may seem counterintuitive, the ASRC is not actually creating signal content or replacing anything lost along the way. A digital audio signal at 44.1 kHz can only contain frequencies up to about 22 kHz. After upsampling this signal to 192 kHz, it still only contains frequencies up to 22 kHz. Nothing is added. So why bother? The benefits, it turns out, are a bit more indirect.
USB Jitter Isolation
The first benefit is isolation from USB interface jitter. The USB data stream can be jittery, meaning the digital bits don't arrive at an exact, fixed rate. The edge of a data bit can arrive slightly early or slightly late, depending on when the host (i.e., the computer) sends it. Normally this wouldn't affect the actual audio signal content - as long as the USB interface chip can determine whether the bit is a 1 or a 0, there's nothing lost. And there's usually plenty of margin to accomplish this with practically no errors. The trouble occurs when the jittery USB data stream is used to generate the clock signals that are needed by the DAC. This is how the jitter can get translated into something potentially audible, such as increased distortion. Using an asynchronous SRC means the clock signals for the DAC don't need to be derived from the USB data stream. Instead, the clock signals are generated by an on-board crystal oscillator which has very low jitter and is very stable. This prevents any jitter on the USB interface from having an effect on the digital-to-analog conversion.
Improved Digital Filter Response
The second benefit has to do with something called an interpolation filter. This is a digital filter which comes into play whenever a digital signal is upsampled. Upsampling creates unwanted, high-frequency signals, and the job of the interpolation filter is to remove these signals. This filter is present not only in the sample rate converter chip, but also in the DAC chip itself. Most modern DAC chips use a technique called sigma-delta modulation, and this involves upsampling the digital signal to a very high sample rate, usually somewhere between 1 and 6 MHz. The interpolation filter in the DAC chip removes anything higher than about half of the original sample rate, while ideally leaving everything in the audio band untouched. So if we start with a digital signal at 44.1 kHz, the interpolation filter must let through audio at 20 kHz while removing everything above about 22 kHz. This requires a steep filter, and the more processing power given to this filter, the better job it can do. The approach taken by some high-end DAC manufacturers these days is to run the DAC chip at a high sample rate and let some other, more powerful processor handle the steep filtering. Running the DAC chip at 192 kHz means its interpolation filter can be configured to have a high cut-off frequency and a very gentle (i.e., non-steep) roll-off. This essentially pushes the effects of the filter away from the audio band.
The UHA760 uses the CS8422 ASRC chip from Cirrus Logic. The interpolation filter in this chip has some advantages over the filter in the CS4398 DAC chip. The CS8422 filter has almost no pre-ringing in the impulse response, for example. The debate over this is ongoing, but DAC designers suspect that pre-ringing can have negative audible effects because it produces a sort of "time smear". Pre-ringing generally does not occur in nature, so it sounds especially strange to our ears. In addition to minimal pre-ringing, the CS8422 also provides near-constant group delay across the audio band, meaning that all frequencies are delayed almost equally through the filter.
Digital Volume Control
The advantage of the digital volume control is that it provides excellent channel-to-channel level matching, better than almost any analog potentiometer. Even a high-quality analog pot can exhibit some channel level mismatch at the ends of the adjustment range (near minimum volume, for example). The digital volume chip in the UHA760 guarantees channel-to-channel matching within +/-0.5 dB at all settings, even minimum and maximum volume settings. My lab measurements show the performance is typically even much better than that. What this all means is that the stereo image in your phones remains stable and centered as you adjust the volume, even if you are using high-sensitivity IEMs which require relatively low levels to drive.
An analog potentiometer is used as the control mechanism on the UHA760, but instead of connecting directly to the amplifier circuitry, the potentiometer provides a signal to the microcontroller which is then converted into the control signal for the digital volume chip.
Some users of the UHA-6S.MKII have commented that the volume knob is a bit too easy to turn, sometimes resulting in unintentional adjustments when the amp is in a pocket or bag. The UHA760 uses a smaller knob and a potentiometer with higher physical resistance, and this should help reduce the chance of an accidental adjustment.
The crossfeed in the UHA760 is an analog type, very similar in design and performance to the crossfeed in the UHA-4. The UHA760 offers two levels of crossfeed along with a true bypass.
Improvements to Power Supply and Turn-On Pop
The UHA760 has some additional circuitry improvements over the UHA-6S.MKII. The UHA760 includes an improved power supply design with better regulation and lower noise. Along with an increase in total power supply capacitance, this new design provides supply voltages to the audio amplifier circuitry which are more stable and less susceptible to ripple caused by high-amplitude transients in the audio signal.
Just prior to the headphone jack of the UHA760 is a mechanical relay. This relay opens automatically during turn-on and turn-off, preventing pops or clicks at the headphones when flipping the power switch.
Available This Fall
I'm currently working on performance characterization of the pre-production version of the UHA760. At this point, all the kinks have been worked out, and the amp is just about ready to begin production. The amp will be available to purchase later this fall. The regular price will be $439. I will be offering a limited-time introductory price of $379.
Following the UHA760
The UHA760 lacks S/PDIF optical and coax inputs, and this is mostly due to a lack of room on the circuit board for the connectors. I know these types of inputs are useful for a lot of folks, so I'm planning a model based on the UHA760 which will be a bit larger in size but will also include S/PDIF inputs, a DAC line output, and socketed op-amps. Support for 24-bit/192-kHz USB streaming is another feature you can expect to see soon.